In contemporary electronic apparatus, the components are usually mounted on a substrate and interconnected by conductors printed or otherwise secured to one or both surfaces of the substrate. The interconnecting conductors are fabricated in a wide variety of processes such as, for example, thick-film fired conductor systems, printed circuit boards, and polymer conductors.
In thick-film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen printed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650.degree. and 900.degree. C. As the temperature rises to the firing temperature, the vehicle is volatilized leaving the metal and binder behind. At the firing temperature, sintering of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate.
The technique used to prepare printed circuit boards can be divided into additive and subtractive technologies. Both procedures require the application of a copper foil over the entire substrate, deposition and removal of a resist, etching of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating. An advantage of this technology is, however, that the resulting circuit boards can be relatively easily soldered. A further advantage is that plated-through holes can be fabricated to interconnect conductors printed on both sides of the boards as well as provide connection land areas for the conductors on the non-component side of the board. These holes are usually plated with copper and/or tin-lead solder.
The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened on to a substrate. The advantage of this system is that the polymer can be cured by means of radiation (e.g. ultraviolet), catalytically or thermally at temperatures which range from room temperature to about 250.degree. C. As a result of this so called "cold processing", it is possible to use very inexpensive substrates such as films of MYLAR (trademark)--polyethylene terephthalate. The mechanism by which conductivity is achieved is supplied entirely by the finite proximity of individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other standard conducting metals tend to oxidize over a period of time and the conductivity between the particles is reduced. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. In addition conductors fabricated using silver polymer ink tend to exhibit characteristics which has made them unusable for printed circuits. Silver migration between closely spread conductors over time creates current leakage problems which may render the circuit useless for its intended use. Silver polymer conductors also exhibit other problems such as dendritic growth, corona degradation, ionic contamination and hydrolytic instability.
It is an object of this invention to provide a process of fabricating printed silver polymer conductors which exhibit greatly improved characteristics over that formed by existing methods.